Watchdog reset: determining cause

[Mike Voytovich]

Hi,

I'd like to determine the cause of a watchdog reset. I was hoping to utilize the WDT event handler (passed in to nrf_drv_wdt_init()) to record the PC of the application code at the time the watchdog timeout occurred, and save that to a word in uninitialized RAM (I am aware of the time constraints in the handler, and the fact that RAM isn't guaranteed to be retained on a WDT reset).

I'd imagine the application PC code must be saved in a register or pushed on to the stack at some point before the event handler is called.

Do you know where I can find this?

[pabigot]

If you're using gcc, aren't deeply coupled to the nRF5 SDK WDT API, and if necessary translate this to plain C, the following also works:

extern "C" {
  /* This little joy loads r0 with the main or process stack pointer then
   * jumps to the systemState watchdog IRQ handler, which will then
   * pull the exception return address out and use that as the value
   * of last_pc showing where the program was when the reset occurred.
   *
   * See https://stackoverflow.com/questions/38618440
   */
  __attribute__((__naked__))
  void WDT_IRQHandler ()
  {
    __ASM (
#if (NRF51 - 0) /* Cortex M0/M1 */
          "mrs r0, msp\n\t"
          "mov r1, lr\n\t"
          "mov r2, #4\n\t"
          "tst r1, r2\n\t"
          "beq 1f\n\t"
          "mrs r0, psp\n"
#else /* Cortex M3/M4 */
          "tst lr, #4\n\t"
          "ite eq\n\t"
          "mrseq r0, msp\n\t"
          "mrsne r0, psp\n\t"
#endif /* ARM Cortext Variant */
          "1:\tldr r1, =%0\n\t"
          "bx r1\n"
          : // no outputs
          : "i" (nrfcxx::systemState::wdt_irqhandler)
          );
  }
} // extern C

void
systemState::wdt_irqhandler (void * sp)
{
    auto statep = nrfcxx::systemState::statep_;
    if (statep) {
      struct exception_frame_type {
        uint32_t r0;
        uint32_t r1;
        uint32_t r2;
        uint32_t r3;
        uint32_t r12;
        uint32_t lr;
        uint32_t pc;
        uint32_t xpsr;
      } const * const efp = reinterpret_cast<const exception_frame_type *>(sp);

      statep->wdt_status = NRF_WDT->REQSTATUS;
      if (efp) {
        statep->last_pc = efp->lr;
        statep->reset_reas_ = state_type::RESET_REAS_WDTBARKED;
      }
    }
    do_reset(false);
}